Materials for electrolytic solutions are widely used in various cells or batteries in which the phenomenon of ionic conduction is utilized, for example in primary cells or batteries, lithium (ion) secondary batteries, fuel cells and other cells or batteries having charge and discharge mechanisms as well as in electrolytic condensers, electric double layer capacitors, solar cells, electrochromic display devices and other electrochemical devices. In these, each cell is generally constituted of a pair of electrodes and an electrolytic solution occurring therebetween and serving as an ionic conductor. Currently used as such ionic conductor are electrolytic solutions prepared by dissolving an electrolyte, such as lithium perchlorate, LiPF6, LiBF4, tetraethylammonium fluoroborate or tetramethylammonium phthalate, in an organic solvent such as γ-butyrolactone, N,N-dimethylformamide, ethylene carbonate, propylene carbonate or tetrahydrofuran. In such ionic conductors, the electrolyte, when dissolved, dissociates into a cation and an anion to cause ionic conduction through the electrolytic solution.
With the spread of laptop computers, palmtop computers, mobile phones, video cameras and other portable electronic devices, lightness and powerfulness have been more and more demanded of cells and batteries in which such ionic conductor materials are used. To cope with the growing demand for such cells and batteries and with the accompanying environmental problems, it is becoming more and more important to develop secondary batteries having a long life.
The form of a typical lithium (ion) secondary battery is schematically shown, in cross section, in FIG. 1. Such a lithium (ion) secondary battery has a positive electrode and a negative electrode each formed of a respective active substance, and an electrolytic constituted of an organic solvent and a lithium salt, such as LiPF6, dissolved as a solute in the solvent forms an ionic conductor between the positive and negative electrodes. In that case, during charging, the reaction C6Li→6C+Li+e occurs on the negative electrode, the electron (e) generated on the negative electrode surface migrates through the electrolytic solution to the positive electrode surface in the manner of ionic conduction. On the positive electrode surface, the reaction CoO2+Li+e→LiCoO2 occurs and an electric current flows from the negative to the positive electrode. During discharging, reverse reactions as compared with those during charging occur, and an electric current runs from the positive to the negative electrode. Lithium (ion) secondary batteries are in use as such secondary batteries, and lithium hexafluorophosphate (LiPF6) is used as an electrolyte salt in most of those lithium (ion) secondary batteries.
However, such an electrolytic solution, which constitutes an electrochemical device, has problems; the organic solvent is readily volatile and has a low flash point, and a liquid leak may readily occur, hence the reliability in long-term use is questioned. Thus, materials which may bring about improvements in these respects have been demanded. Lithium hexafluorophosphate (LiPF6) and like lithium salts are compounds low in thermal stability and readily hydrolyzable. Lithium (ion) batteries in which such salts are used become very complicated in structure and are manufactured by a very expensive process. Furthermore, owing to the sensitivity of such lithium salts, the life and performance of lithium (ion) batteries may be shortened or lowered and, under extreme conditions, such as elevated temperatures, the use thereof may not be allowed.
In the prior art, Koura et al.: J. Electrochem. Soc. (USA), (1993) vol. 140, page 602 discloses the application of an ordinary temperature molten salt, which occurs as a liquid at room temperature. C. A. Angell et al.: Nature (UK), (1993) vol. 362, page 137 discloses, as ordinary temperature molten salts, complexes between an N-butylpyridinium, N-ethyl-N′-methylimidazolium or like aromatic quaternary ammonium halide and an aluminum halide as well as mixtures of two or more lithium salts. However, the former complexes have a problem of corrosion by halide ions, while the latter complexes are thermodynamically unstable supercooled liquids and have a problem in that they solidify with the lapse of time.
On the other hand, imidazolium or pyridium salts of tetrafluoroborate anion, bistrifluoromethanesulfonylimide anion or the like are relatively stable from the electrical viewpoint and have recently become targets of close investigation. However, they are unsatisfactory in performance, for example ionic conductivity and, due to their containing fluorine, they cause the problem of electrode corrosion, for instance. Thus, there is room for contrivance for providing ionic conductor materials improved in fundamental performance.
Douglas R. MacFarlane et al., Chem. Commun. (UK), (2001) pp. 1430-1431, who studies the thermal characteristics, viscosity and qualitative potential stability of N-alkyl-N-methylpyrrolidinium or 1-alkyl-3-methylimidazolium dicyanamide salts, disclose that such dicyanamide salts are useful as low-viscosity ionic liquids. However, there is no disclosure about the technology of applying such dicyanamide salts as ionic conductor materials in electrochemical devices. Thus, there is room for contrivance for providing ionic conductor materials improved in fundamental performance.
J P Kohyo 2002-523879 (pages 1-7 and 30-43), which is concerned with cyano-substituted salts including cyano-substituted methides and amides, discloses an electrolyte comprising a matrix material and at least one salt selected from the group consisting of N-cyano-substituted amides, N-cyano-substituted sulfonamides, 1,1,1-dicyano-substituted sulfonylmethides and 1,1,1-dicyanoacylmethides. For obtaining this electrolyte, a powder form of such a salt is prepared and this is dissolved in an organic solvent, which is a matrix material, to give a liquid electrolyte, or used to give a solid polymer electrolyte. In the pamphlet of Laid-open International Patent Application WO 01/15258 (pages 14-17), which is concerned with solid conductive materials containing an ionic dopant to serve as a conductive species in an organic matrix, there is disclosed, among others, a material in which N-methyl-N-propylpyrrolidinium dicyanamide salt is used as the organic matrix and LiSO3CF3 as the ionic dopant. However, these technologies do not include any disclosure about the application of such a compound in salt form as a material in electrolytic solutions to serve as good ionic conductors in electrochemical devices and about the use of such salt itself in a liquid form. Thus, there is room for contrivance for utilizing compounds having ionic conductivity as materials constituting electrolytic solutions capable of exhibiting excellent basic performance.
J P Kohyo 2000-508677 (pages 1-12), which is concerned with ionic compounds containing an anionic moiety bound to a cationic moiety M+m, discloses that those ionic compounds in which the cationic moiety M is hydroxonium, nitrosonium NO+, ammonium NH4+, a metal cation having a valency of m, an organic cation having a valency of m or an organometallic cation having a valency of m and the anionic moiety corresponds to the formula RD—Y—C(C≡N)2− or Z—C(C≡N)2− can be used as ionic conductor materials, for instance. In such ionic compounds, however, the atom (C) is the only anion-constituting element in the anionic moiety. Thus, there is room for contrivance for modifying them to provide materials suited for use in constituting electrolytic solutions showing excellent basic performance.
U.S. Pat. No. 4,505,997 and WO 92/02966 disclose the technology of using lithium bis(trifluoromethylsulfonyl)imide and lithium tris(trifluoromethylsulfonyl)methanide salts as electrolyte salts in cells or batteries. These salt compounds both show a high level of anode stability and form highly conductive solutions together with an organic carbonate. However, lithium bis(trifluoromethylsulfonyl)imide does not passivate the aluminum metal functioning as a cathode terminal conductor to a satisfactory extent. Thus, there is room for contrivance in this respect.
Meanwhile, J P Kohyo 2002-523879 (pages 1-7, 20-22 and 30-43), which is concerned with electrolytes comprising an N-cyano-substituted amide salt or the like and a matrix material, discloses that another known conductive salt may be added to cell or battery electrolyte compositions. However, none of these prior art organic materials is electrochemically stable even under application of a high voltage. Thus, there is room for contrivance for modifying them to give materials suited for use as ionic conductive organic materials.